The invention relates to a base for an encapsulating case for components having coplanar electrodes, with means for the precise positioning of the components in order to ensure a good reproducibility of the mounting of the components on the base and the electrical characteristics, particularly in the field of ultra-high frequencies.
In this type of application, the components are encapsulated or enclosed in cases which very often contain a plurality of components, such as for example two transistors and two capacitors hard soldered or brazed to the same base. Thus, in the field of ultra-high frequencies it is necessary to match the circuit in question in order to match the impedances and it is known that the length of the connections increases in importance with the frequency. The accuracy required is sufficiently high to take into consideration the positioning of the components on the base to which they are fixed.
An encapsulating case with coplanar electrodes is constituted by the following elements:
a mounting stud, which also serves to dissipate the heat given off during the operation of the semiconductor chips and made from a metal or alloy often copperbased; PA1 an insulating plate, preferably of beryllium oxide due to the very good thermal conductivity of this compound, the insulating plate incorporating localized metallization regions to which are hard soldered the active components such as transistors and passive components such as capacitors or the output terminals of the device; PA1 a connecting grid cut from a single metal plate and which forms the input and output terminals of the device; PA1 a cover which both seals and protects the device.
The difficulties to solve are essentially linked with the very precise positioning of the active and passive components on the base of the encapsulating case, the base being formed by the mounting stud, the insulating plate brazed to the latter and the connecting grid brazed on the insulating plate.
This positioning is vital for a number of reasons. If the device only has a single active component and the latter is, for example, an ultra-high frequency transistor, it is still vital that it is positioned with a great deal of accuracy in order that all the transistors of a same production batch have the same electrical characteristics at the end of manufacture, so that they will be interchangeable in ultra-high frequency equipments in which they are to be used. If the device comprises a plurality of active and passive components, e.g. because the power level required necessitates the parallel connection of several transistor chips, the aforementioned reasons still apply, but in addition it is important that all the components on the same base are reciprocally positioned with a great deal of accuracy in order to obtain a good overall reproducibility of the mounting arrangement.